In industrial applications, waveguide structures of this kind are used to create phased arrays in the microwave range. Due to the phase gradient φ between the input signals of adjacent antenna elements, their output signals are subject to phase lead/lag, and in consequence the azimuthal angle of the resulting phase front of the antenna output signal wave is modified.
There are many related-art civil and military radar and communications applications for microwave antennas having electronically swivelable or switchable lobes. One example is automotive radar systems for adaptive cruise control (ACC), which typically use multi-lobe monopulse radar. Herein, one or a plurality of antennas is used to create a plurality of lobes in the azimuthal plane, which overlap in pairs in some areas. In addition to ACC, future automotive applications include low-speed follow, stop-and-go, assisted reverse/park, blind-spot monitor, collision detection with collision-avoidance means or means for limiting collision severity via driving maneuvers, enhancing or deploying restraining devices, airbags, etc. The beam lobes in automobile radar sensors currently commercially available are usually created using a dielectric lens. Current research is geared towards a completely planar arrangement, i.e., radar sensors having a planar antenna and a planar waveguide lens structure for beam shaping connected in series, which is advantageous from a cost and space standpoint.
In industrial applications, use of planar waveguide lens structures is known heretofore, e.g., a Rotman lens, which at its outputs creates a phase gradient that is dependent on the selected input. The antenna elements are coupled to the outputs of a lens structure of this kind, and thus, depending on the selected input, create a beam lobe having beam deflection that is a function of the phase gradient. Rotman lenses have good focusing properties, and the arrangement is flexible based on the desired phase gradients at the antenna ports. In industrial applications, lens structures of this kind are used in conjunction with a planar antenna having a plurality of fixed beam lobes, using planar microstrip technology. Herein, the elements of the lens are arranged as planar elements of a microstrip circuit on a microwave substrate, e.g., ceramic material, glass or filled plastics.
The basic arrangement of a Rotman lens is shown in FIG. 1. A parallel plate guide 1 is supplied on one side via beam lobe ports 2, which are connected, via microstrips 3 and, if necessary, via a changeover switch for selecting a beam lobe, to a send/receive circuit (not shown). In parallel plate guide 1, waves propagate to antenna ports 4. At antenna ports 4, the wave of parallel plate guide 1 passes to microstrip conductors 5, via which antenna elements 6 are coupled. Microstrip conductors 5 between antenna ports 4 and antenna elements 6 are arranged as equalizing conductors, having a variable length from the middle of parallel plate guide 1 outward. The contoured shape of parallel plate guide 1 and the lengths of equalizing conductors 5 determine the respective signal path length. They are arranged in such a way that in the case of a centrally positioned beam lobe port a phase gradient of zero is obtained at antenna elements 6, and the maximum predefined phase gradient is obtained in the case of the outermost beam lobe port.
The lens structure just described has various disadvantages which may make it unsuitable for industrial applications involving radar sensors. Losses in the lenses, in particular due to the equalizing conductors, are relatively high. Furthermore, parallel plate guides and equalizing conductors require a relatively large amount of space. As a general rule, there is a relatively large amount of irradiation loss at the sides of the lens structure and from the parallel plate guide. Moreover, when a Rotman lens is used the beam lobe ports are at a significant distance from the antenna. This means the sensor has to be relatively long in the direction of elevation, which is not favorable for automotive applications.